The present invention relates to a color cathode ray tube, a circuit for driving a color cathode ray tube, a color image reproducing device employing the circuit and a color image reproducing system including the color image reproducing device, which are capable of switching between displaying a high-brightness image and displaying a high-definition image.
As for electronic apparatuses employing color cathode ray tubes, television receivers and display monitors of terminals for information equipment represented by personal computers are placed as separate articles of commerce on the market.
The display monitors for information terminals are required to provide high-definition images, and it is essential that they have high resolution capability. Therefore the display monitors needs to be driven at high frequencies (high deflection frequencies), and produce sufficiently small electron beam spots. Priority is given to reduction of electron beam spots, and as a result their display brightness and contrast are set to be lower than those of the television receivers.
On the other hand, first of all, high brightness and high contrast are required of the color television receivers so as to present realism in their scenes, and since the frequencies are prescribed by the color television systems such as NTSC, PAL and SECAM, the degree of image definition is not valued so highly as in the case of the display monitors for information terminals. As a result scene brightness and display contrast have priority over electron beam spot diameters, and therefore it is important to obtain large currents.
In present systems which includes a display monitor for an information terminal and is also configured so as to be capable of receiving television broadcasts by using the display monitor, when they receive television broadcast, the display monitor have to increase electron beam spot diameters compared with those of the display monitor used for the information terminal, or reduce scene brightness and display contrast compared with those of color television receivers.
FIG. 15 is an illustration of an example of a relationship between cathode cutoff voltages and cathode currents with a fixed drive voltage of 40 V in color cathode ray tubes, with the abscissa representing cathode cutoff voltages Ekco (V) and the ordinate representing cathode currents Ik (mA). A drive voltage Ed is defined as a difference (Ekcoxe2x88x92Ek) between a cathode voltage Ek for producing the amount of an electron beam current corresponding to a video signal and a cutoff voltage (Ekco), as explained subsequently in connection with FIG. 5. In one color cathode ray tube, when a voltage Ec1 applied on the first grid electrode is fixed, the cutoff voltage Ekco increases as the voltage Ec2 on the second grid electrode is increased. To increase a cathode current with the fixed drive voltage, the cutoff voltage Ekco needs to be lowered, in other words, the voltage Ec2 needs to be lowered.
FIG. 16 is an illustration of an example of a relationship between cutoff voltages and electron beam spot diameters for color cathode ray tubes, with the abscissa representing cathode cutoff voltages Ekco (V) and the ordinate representing electron beam spot diameters (mm) at the 10% intensity profile.
As is apparent from FIG. 16, to produce electron beam spot diameters corresponding to a high-resolution display, the cutoff voltages Ekco needs to be sufficiently high, and hence the second grid electrode voltage Ec2 needs to be sufficiently high.
FIG. 17 is an illustration of examples of a conventional color cathode ray tube and a conventional driving circuit for diving the color cathode ray tube used in conventional color TV receivers or conventional display monitors of information terminals. Reference numeral 20 denotes a color cathode ray tube, 21, 21xe2x80x2 and 21xe2x80x3 are cathodes for red, green and blue electron beams, respectively, 22 is the first grid electrode, 23 is the second grid electrode, 24 is the third grid electrode, and 25, 26 and 27 are output transistors for red, green and blue signals, respectively. The fourth grid electrode and grid electrodes succeeding it in the electron gun are omitted in FIG. 17.
Reference numeral 1 denotes a variable resistor for setting cutoff voltages of three electron guns by adjusting a voltage applied on the second grid electrode 23, and 2, 3 and 4 are variable resistors for adjusting voltages applied on emitters of the output transistors 25, 26 and 27 for the red, green and blue signals, respectively, such that voltages on the cathodes 21, 21xe2x80x2 and 21xe2x80x3 are adjusted independently of each other, thereby to absorb differences in cutoff voltages among the three electron guns. Reference numerals 5 and 6 are variable resistors for adjusting the magnitude of drive voltages applied on cathodes 21 and 21xe2x80x3, respectively, and 8 is a variable resistor for adjusting a voltage applied on the third grid electrode 24.
The color cathode ray tube 20 employs three electron guns, and the three electron guns differ in characteristics from each other because of a slight structural variability among the three assembled electron guns, and further, red, green and blue color phosphors of the color cathode ray tube 20 differ in luminous efficiency, and therefore voltages applied on the three cathodes are adjusted to compensate for the differences in the characteristics such that the three electron beams are adjusted in amount and thereby the three colors produced by the three electron beams balance with each other regardless of the scene brightness.
Generally the three electron guns for the three electron beams, respectively, are fabricated as an integral structure, and the electrodes other than the cathodes 21, 21xe2x80x2 and 21xe2x80x3 are fabricated for the three electron beams in common. Therefore, white balance adjustment for the three colors is made mainly by adjusting the cathode voltages.
In the circuit configuration shown in FIG. 17, a combination of the maximum magnitudes of the voltages on the second grid electrode 23, the three cutoff voltages and the magnitudes of the three drive voltages is necessarily determined, and hence can not be changed freely.
Consequently, a high-definition display required of the display monitors of information terminals and a high-brightness and high-contrast display required of the color television receiver could not be realized by one apparatus.
The conventional technique of this kind is disclosed in Japanese Patent Application Laid-open No. Hei 9-191462 assigned to the assignee of the present invention.
As described above, with the conventional technique, it was impossible to switch between a high-brightness display and a high-definition display by using one driving circuit for a color cathode ray tube, and therefore it has been a problem to make it possible to perform two functions required of a display monitor of an information terminal and a color television receiver, respectively, by using one apparatus.
It is an object of the present invention to provide a color cathode ray tube, a circuit for driving a color cathode ray tube, a color image reproducing device employing the circuit and a color image reproducing system including the color image reproducing device, which make it possible to switch between a plurality of driving modes such that one apparatus can perform two functions required of display monitors of various information terminals and color television receivers for various color television systems, by solving the above problems with the conventional technique.
The following are representative ones of a color cathode ray tube, a circuit for driving a color cathode ray tube, a color image reproducing device employing the circuit and a color image reproducing system including the color image reproducing device, in accordance with the present invention.
In accordance with an embodiment of the present invention, there is provided a color cathode ray tube having a phosphor screen and an electron gun, the electron gun comprising: a triode section including three transversely-spaced in-line cathodes adapted to be supplied with video signals, a first grid electrode and a second grid electrode arranged in the order named; and a plurality of electrodes for focusing three electron beams emitted from the triode section onto the phosphor screen, wherein the following inequalities are satisfied: Exe2x89xa61.4Axe2x88x920.2Bxe2x88x922.7Cxe2x88x922D, Axe2x89xa60.35 mm, where A (mm) is a diameter of an electron-beam transmissive aperture in the first grid electrode, B (mm) is a diameter of an electron-beam transmissive aperture in the second grid electrode, C (mm) is a thickness of a portion of the first grid electrode immediately surrounding the electron-beam transmissive aperture in the first grid electrode, D (mm) is a spacing between the three transversely-spaced in-line cathodes and the electron-beam transmissive aperture in the first grid electrode, and E (mm) is a spacing between the first grid electrode and the second grid electrode.
In accordance with another embodiment of the present invention, there is a driving circuit for driving a color cathode ray tube including a voltage-setting circuit for setting voltages to be applied on cathodes and electrodes of the color cathode ray tube having three cathodes for emitting three electron beams and adapted to be supplied with video signals, a first grid electrode for the three electron beams in common, and a second grid electrode for the three electron beams in common, arranged in the order named, the voltage-setting circuit comprising: a circuit configured to provide a plurality of combinations of three cathode bias voltages to be applied on the three cathodes, respectively, a first grid electrode voltage to be applied on the first grid electrode and a second grid electrode voltage to be applied on the second grid electrode; and a switching circuit for selecting one of the plurality of combinations such that a voltage difference between the second grid electrode voltage and the first grid electrode voltage increases when a horizontal deflection frequency of the three electron beams is increased.
In accordance with another embodiment of the present invention, there is a driving circuit for driving a color cathode ray tube including a voltage-setting circuit for setting voltages to be applied on cathodes and electrodes of a three-electron-beam color cathode ray tube having three cathodes for emitting three electron beams and adapted to be supplied with video signals, a first grid electrode for the three electron beams in common, and a second grid electrode for the three electron beams in common, arranged in the order named, the voltage-setting circuit comprising: a circuit configured to provide a plurality of combinations of three cathode bias voltages to be applied on the three cathodes, respectively, and a second grid electrode voltage to be applied on the second grid electrode, with a first grid electrode voltage to be applied on the first grid electrode being fixed; and a switching circuit for selecting one of the plurality of combinations such that the second grid electrode voltage increases when a horizontal deflection frequency of the three electron beams is increased.
In accordance with another embodiment of the present invention, there is a driving circuit for driving a color cathode ray tube including a voltage-setting circuit for setting voltages to be applied on cathodes and electrodes of the color cathode ray tube having three cathodes for emitting three electron beams and adapted to be supplied with video signals, a first grid electrode for the three electron beams in common, and a second grid electrode for the three electron beams in common, arranged in the order named, the voltage-setting circuit comprising: a circuit configured to provide continuously adjustable voltages to at least one of the three cathode, and at least one of the first grid electrode and the second grid electrode; and a voltage control circuit for controlling the continuously adjustable voltages such that a voltage difference between the second grid electrode and the first grid electrode increases when a horizontal deflection frequency of the three electron beams is increased.
The present invention is not limited to the above configurations or the configurations of the embodiments described subsequently, and various changes and modifications may be made without departing from the spirit and scope of the present invention.